Increase control register count

[jbush001]

Currently the control register index uses a 5 bit register field for the R format instruction. Change to instead use the immediate field of an immediate instruction, expanding total available registers.

(issue #145 will do this more cleanly, this is an interim change before that one)

[jbush001]

The motivation for this is that I want to add a bunch of new control registers and space is quickly running out (there are already 19 used, and there are features that want to at least another 17)

• Want to move performance counters out of memory mapped space into CR space (#147) +8
• Adding inter-thread interrupts (#61) +1
• Adding instructions to query machine information (#86) +1-4
• Widening the cycle counter register (#100) +1
• Hardware watchpoints (#101) +2
• Performance counter interrupts (#72) +4?

There are plenty of other functions in the future that could be useful to put into control registers.